Radical polymerisation initiators for light-curable dental materials

ABSTRACT

The invention provides a light-curable dental material comprising a polymerizable compound having at least one ethylenically unsaturated bond and a hexaaryl bisimidazole as a photopolymerisation initiator.

FIELD OF THE INVENTION

The present invention relates to light-curable dental materials, such asdental composites and dental adhesives, comprising at least onepolymerizable compound having at least one ethylenically unsaturatedbond and a hexaarylbisimidazole, e.g. as a photopolymerizationinitiator. The Invention further relates to the use of ahexaarylbisimidazole polymerization initiator in light-curable dentalmaterials. The invention also relates to cured dental materialsobtainable by light-curing the light-curable dental materials of theinvention.

BACKGROUND OF THE INVENTION

Radical polymerisation of ethylenically unsaturated compounds is widelyused to harden dental materials such as dental filling materials. Inlight-curable dental materials, the ethylenically unsaturated compoundsare activated to be polymerisable by the application of light, sincethis allows a “command cure”. By this means the ethylenicallyunsaturated substances remain workable for an indefinite time, but canbe cured at will in a short time by the application of light. Generallythe actinic light has a wavelength between about 200 nm and 700 nm, andmore often between about 300 nm and 600 nm. In light-curable dentalcompositions, camphor quinone which allows photopolymerization withvisible light is almost exclusively used in combination with an amine asa reductant (co-initiator).

In many cases, it is necessary or convenient to apply the activateddental material or to model it under ambient light. Very often arelatively strong ambient light is needed so that the modelling orapplication can be carried out with the necessary precision, which leadsto several conflicts. If sufficient ambient light is present foraccurate and precise use of the dental material, the life time of theactivated material is reduced, and the time available for the modellingor application is also reduced. If less ambient light is present, thelife time of the material and the time available to model it (“workingtime”) is increased, but the precision of the modelling is reduced. Thelifetime of the material towards ambient light may be increased by, forinstance, reduction in the concentration of initiators, or increasingthe amount of polymerisation inhibitor present. However, both of thesemeasures can lead to a decrease in the physical properties of thelight-cured dental material.

It is therefore an object of the invention to provide a light-curabledental material having a longer working time under ambient light withoutcompromising the physical properties of the cured dental material.

SUMMARY OF THE INVENTION

The invention provides a light-curable dental material comprising

-   (i) a polymerizable compound having at least one ethylenically    unsaturated bond and-   (ii) a hexaaryl bisimidazole.

The invention further provides a light-curable dental materialcomprising

-   (i) a polymerizable compound having at least one ethylenically    unsaturated bond;-   (ii) a hexaaryl bisimidazole; and-   (iii) at least 10 weight-% based on the total weight of said    light-curable dental material of a solid particulate filler.

The hexaaryl bisimidazole may be used as a polymerization initiator forpolymerizing said polymerizable compound. The invention also provides alight-curable dental material comprising a hexaaryl bisimidazole (e.g.as a polymerization initiator or inhibitor) and a second compoundcapable of acting as a polymerization initiator. Said second compoundmay be an α,β-diketone such as camphor quinone or acetophenone, aphosphine oxide, or a compound containing a single carbonyl group suchas benzophenone, benzoin, or benzoin methylether.

The invention further provides a light-cured dental material obtained orobtainable by light curing the light-curable dental materials of theinvention. Further, a use of a hexaaryl bisimidazole in a light-curabledental material is provided, notably a use of a hexaaryl bisimidazole asa photopolymerization initiator.

In the present invention, it has surprisingly been found that longerworking times can be obtained using hexaaryl bisimidazole as apolymerization initiator in light-curable dental materials compared toprior art light-curable dental materials that use almost exclusively thecamphor quinone/amine initiator system. Even more surprisingly, thephysical properties of the light-cured dental materials are notcompromised to a significant extent. Generally, the physical propertiesof the light-cured dental materials are even improved compared tolight-curable dental materials that use the camphor quinone/amineinitiator system. Further, it has been found that the working times ofconventional light-curable dental materials containing the camphorquinone/amine initiator system can be extended if the hexaarylbisimidazole of the invention is additionally included.

The hexaaryl bisimidazole used as a polymerization initiator in thepresent invention has the general structure of formula (I)

Ar₃T-TAr₃  (I)

wherein each T represents an imidazole moiety and each Ar stands for anoptionally substituted aryl group. Multiple Ar in compounds of formula(I) may be the same or different. The substitution on different groupsAr may be the same or different. The aryl groups in the hexaarylbisimidazole compounds used in the present invention may be substitutedor unsubstituted phenyl, naphthyl, phenanthryl, 2-pyridyl, 3-pyridyl,4-pyridyl, or quinolinyl groups. Substituted or unsubstituted phenyl,naphthyl, or phenanthryl groups are preferred, and substituted orunsubstituted phenyl and naphthyl groups are most often used. In oneembodiment, all aryl groups of the hexaaryl bisimidazole compounds aresubstituted or unsubstituted phenyl groups. The optional substituents ofgroups Ar are as described below with regard to Ar¹, Ar², and Ar³.

The hexaaryl bisimidazole may be a homodimer or a heterodimer obtainableby oxidation of any of the triarylimidazoles represented by thefollowing formula (II):

In formula (II), Ar¹, Ar², and Ar³ are aryl groups as defined above.Ar¹, Ar², and Ar may be the same or different and each represents anoptionally substituted aryl group. Multiple Ar¹, Ar², or Ar³ on the samehexaarylbisimidazole may be the same or different. In one embodiment,Ar¹, Ar² are optionally substituted phenyl groups and Ar³ is anoptionally substituted phenyl or naphthyl group.

Each substituted aryl group may have 1 to 5, preferably 1 to 3,substituents selected from the group consisting of C₁₋₆-alkyl,C₁₋₆-alkoxy, hydroxy, halo, cyano, nitro, nitroso, mercapto, carboxyl,sulfonate, thiol, amino, phenyl, pyridyl, and trifluoromethyl. TheC₁₋₆-alkyl groups and the C₁₋₆-alkyl groups of the C₁₋₆-alkoxy groupsmay by linear or branched C₁₋₆-alkyl groups or C₃₋₆ cyclic alkyl groups.Examples of C₁₋₆-alkyl groups are methyl, ethyl, propyl (such asi-propyl or n-propyl), butyl (such as n-butyl, isobutyl, tert-butyl orsec-butyl), pentyl, and hexyl. “Halo” stands for the class consisting offluoro, chloro, bromo or iodo.

The hexaarylbisimidazole compounds can be prepared as known in the artby oxidation of triarylimidazole compounds. A procedure for thepreparation of hexaarylbisimidazoles is described for example in DE 1470154 and in U.S. Pat. No. 4,459,349.

The structure of the hexaarylbisimidazole compounds to be used in thepresent invention may be represented by the following formula (III):

wherein Ar¹, Ar² and Ar³ are as defined above and multiple Ar¹, Ar² orAr³ on the same hexaarylbisimidazole compound may be the same ordifferent. There are various possibilities for the bonding between thetwo imidazole moieties. Different atoms within each imidazole moiety maybe involved in this bond. This bond may be between two imidiazolonitrogen atoms, between a nitrogen atom of one imidazolo moiety and acarbon atom of the other imidazolo moiety, or between two carbon atomsof the imidazolo moieties.

The general structure of formula (II) is shown with the two moietiesjoined by a fine. In fact, two or more alternatively linkedhexaarylbisimidazole compounds may co-exist e.g. in an equilibrium.

An example of a hexaarylbisimidazole compound usable in the invention isshown in formula (IV), known as2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,2′-biimidazole, whichis also identified by the abbreviations BCIM, o-Cl HABI or 2-Cl HABI.HABI stands for “hexaarylbisimidazole”.

A further example of a commercially available HABI compound is shown inthe following formula (V), and illustrates the possibility of obtaininga HABI structure with different substituents on each imidazole moiety.The compound of formula (V) is known as2,2′,4-tris(2-chlorophenyl)-5-(3,4-dimethoxyphenyl)-4′,5′-diphenyl-1,1′-biimidazole,abbreviated to TCDM-HABI or CZ-HABI by various manufacturers. In factthis HABI probably exists as an equilibrium mixture of the compounds ofthe compound class shown, together with homo-dimers of the two imidazolestructures.

By adjusting the nature of the aryl groups and their substituents, theproperties and stability of the hexaarylbisimidazole and the resultingradicals can be adjusted. The stability of the radicals produced bysplitting the hexaarylbisimidazole into two triarylimidazole radicalscan be estimated by measuring the time needed for the colour of theradical to fade, since this is related to the time needed for the twomoieties to recombine or be otherwise annihilated. The less stable aradical is, the higher is its fading rate and the shorter is itslifetime. The values for the “fading rate” given in Table 1 are obtainedfrom U.S. Pat. No. 3,784,557, and were measured by first illuminating asolution of the bisimidazole in benzene, and then measuring theabsorption of the solution at the absorption maximum at suitable minuteintervals. A plot of 1/(abs-abs∞) against time gives a graph, the slopeof which is the “fading rate”, where abs is the absorption of thesolution at time t and abs∞ is the absorption of the solution afterleaving it in the dark over night. The absorption maximum and theextinction coefficient can be adjusted by altering the type and positionof the substituents, as is illustrated by values drawn from theliterature for a range of substituents, and shown in. Obviously, furthercombinations and variations of the substituents are possible, and thosegiven earlier or shown in Table 1 are for illustrative purposes only.

The substituents defined in Table 1 relate to the homodimer of formula(VI) below. Numbers in Table 1 indicate the position of the indicatedsubstituent. “0” indicates that the indicated substituent is notpresent. “2-phenyl substituent 1” relates to the 2-phenyl group on bothtriarylimidazole moieties. “2-phenyl substituent 2” relates to apossible second substituent on the 2-phenyl group on bothtriarylimidazole moietes. In all cases of Table 1, substituents P″ and“Q” are present in both triarylimidazole moieties of thehexaarylbisimidazole of formula (VI).

TABLE 1 2-phenyl 2-phenyl substituent 1 substituent 2 4-phenyl 5-phenylradical position position substituent substituent fading rate absorptionextinction substituent X, Y, or Z X, Y, or Z position P position Q abs⁻¹· min⁻¹ max (nm) coefficient methoxy 4 0 4 4 0.0077 methoxy 4 3 0 00.091 methoxy 4 0 0 0 0.092 610 6.9 chloro 0 0 2 0 0.2 530 2.94 methyl 40 0 0 0.22 bromo 4 0 0 0 0.36 560 3.9 chloro 4 0 0 0 0.41 570 4.49methoxy 4 0 4 0 0.43 none 0 0 0 0 0.46 550 3.28 fluoro 4 0 0 0 0.46methoxy 2 0 2 4 0.5 methoxy 2 4 0 0 0.82 620 8.13 methoxy 2 0 4 0 1.61methoxy 2 3 0 0 3.42 methyl 2 0 0 0 3.5 fluoro 2 0 0 0 6.2 chloro 2 0 00 7.3 540 2.8 bromo 2 0 0 0 7.4 560 4.07 methoxy 2 0 0 0 7.7 chloro 2 40 0 17 558 4.6 chloro 3 0 0 0 540 2.9 chloro 0 0 2 2 560 1.32 nitro 3 00 0 540 1.14 nitro 4 0 0 0 530 1.38 cyano 4 0 0 0 560 2.19 —CF₃ 4 0 0 03.22

Herein, the term “hexaaryl bisimidazole” includes salts of the hexaarylbisimidazole.

Based on the total weight of all polymerizable compounds having at leastone ethylenically unsaturated bond used in the light-curable dentalmaterial, the hexaaryl bisimidazole may be used in an amount of from0.01 to 2.0, preferably from 0.05 to 1.0, and most preferably from 0.1to 0.7 weight-%. It is possible to combine two or more differenthexaaryl bisimidazole compounds in the light-curable dental material ofthe invention. In the latter case, the amounts given apply to the sum ofall hexaaryl bisimidazoles used.

The light-curable dental material of the invention generally furthercomprises a co-initiator selected from the following compound classes:thiols, heteroaromatic thiols, benzothiazoles, benzooxazoles, aminessuch as tertiary amines, alcohols, thiocarboxylic acids. Specificexamples of co-initiators are 3-mercapto-4-methyl-4H-1,2,4-triazole or2-mercaptobenzimidazole. The co-initiator may be used in an amount offrom 0.01 to 2.0, preferably of from 0.05 to 1.5, more preferably 0.05to 1.0, more preferably from 0.1 to 1.0 and most preferably from 0.1 to0.7 weight-% based on the total weight of all polymerizable compoundshaving at least one ethylenically unsaturated bond used in thelight-curable dental material. It is possible to combine two or moredifferent co-initiators in the light-curable dental material of theinvention. In the latter case, the amounts given apply to the sum of allco-initiators used.

Light sources as commonly used by dentists may be used for polymerizingthe light-curable dental materials of the invention, such as those usedfor polymerizing light-curable dental materials containing camphorquinone as photoinitiator. Such light sources emit cold blue lightwithin the wavelength range of from 400 to 550 nm. A suitable lightsource is the Spectrum Lite™ from Dentsply.

The light-curable dental material of the invention may also comprise asensitizer to extend the useful range of absorption wavelengths.Suitable sensitizers include but are not limited to coumarin andderivatives of coumarin such as 7-diethylamino-3-thenoylcoumarin,7-diethylamino-4-methylcoumarin, 3-Benzoyl-7-methoxycoumarin,2,3,5,6-1H,4H-tetrahydro-8-methylquinolizino-[9,9A,1GH] coumarin,derivatives of cyclopentanone such as JAW (2,5-Bis[(1H, 5Hbenzil[I,J]quinolizin-1-yl)methylene]-cyclopentanone),2,5-benzylidenecyclopentanone, DEAW dye(cyclopentanone-2,5-bis[[4-diethylamino)phenyl]methylene-2E,5E), saltssuch as bis(4-t-butyl phenyl) iodonium hexafluorophosphate, borate ion,cyanine dyes such as a copper(II) phthalocyanine dye, squarilium and itsderivatives, cyclobutenediylium1,3-bis[(1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2,4-dihydroxy-bisinner salt, N-phenylglycine, eosin, methylene blue, derivatives oftropinone or tropanone such as tropanone 2,4-bis[2-(3-ethyl-2(3H)-benzoxazylidene) ethylidene], aryl ketones, anddialkylaminoaldehydes.

The light-curable dental material of the invention generally furthercontains a polymerization inhibitor. Examples of polymerizationinhibitors are phenolic compounds such as BHT or stable radicals such as2,2,4,4-tetramethylpiperidinyl-1-oxy radical,2,2-diphenyl-1-picrylhydrazyl radical, galvinoxyl radical, ortriphenylmethyl radical. It is possible to combine two or more differentpolymerization inhibitors in the light-curable dental material of theinvention. The amounts of the polymerization inhibitors is chosen suchthat a useful working time is achieved.

The light-curable dental material may be a dental composite material, adental glass ionomer, a dental sealant, a dental adhesive, a adhesionpromoter, an adhesion preventing material, a cement, a crown-formingmaterial, or an impression material. In an important embodiment, thelight-curable dental material is a filler-containing light-curabledental material that contains at least 10 weight-% of a solidparticulate filler. Depending on the type of the light-curable dentalmaterial, a suitable solvent may be present as generally known in theart. The utility of the bisarylimidazole initiators of the inventiondoes not depend on the particular type or purpose of the light-curabledental material.

The light-curable dental material of the invention contains at least onepolymerizable compound having at least one ethylenically unsaturatedbond. Said polymerizable compound having at least one ethylenicallyunsaturated bond may be a polymerizable (meth)acrylic monomer having a(meth)acryl moiety.

In one embodiment, the light-curable dental material contains at leastone polymerizable (meth)acrylic monomer having at least twopolymerizable groups, such as at least two (meth)acrylic moieties, forallowing cross-linking upon light-curing. Said polymerizable(meth)acrylic monomer having at least two polymerizable groups has atleast two (i.e. two, three, four or more) polymerizable groups. Suchpolymerizable monomers are known to the skilled person from conventionaldental materials. Examples are di(meth)acrylates of alkanediols oralkanediamines and other poly- or multifunctional (meth)acrylates;urethane di(meth)acrylates which may be reaction products of 2 mol of ahydroxyalkyl (meth)acrylate with 1 mol of diisocyanate. Specificexamples include 1,3-butylene glycol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, neopentyl glycol di(meth)acrylate, bisphenol Adi(meth)acrylate, bisphenol A glycidyl di(meth)acrylate, ethyleneoxide-modified bisphenol A di(meth)acrylate, ethylene oxide-modifiedbisphenol A glycidyl di(meth)acrylate,2,2-bis(4-methacryloxypropoxyphenyl)-propane,7,7,9-trimethyl-4,13-dioxa-3,14-dioxo-5,12-diazahexadecane-1,1,6-dioldi(meth)acrylate (UDMA), neopentyl glycol hydroxypivalatedi(meth)acrylate, caprolactone-modified neopentyl glycol hydroxypivalatedi(meth)acrylate, trimethylolethane di(meth)acrylate, trimethylolpropanedi(meth)acrylate, and the like. 2, 3 or more different polymerizablemonomers may be used as a mixture.

Examples of polymerizable (meth)acrylic monomers having three or morepolymerizable groups are trimethylolmethane tri(meth)acrylate,trimethylolethane tri(meth)acrylate, trimethylolpropanetri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritoltri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol tetra(meth)acrylate, dipentaerythritolhexa(meth)acrylate, and the like.

Further examples of polymerizable compounds having at least oneethylenically unsaturated bond are polymerizable monomers like(meth)acrylic monomers having one (meth)acrylic moiety like methyl(meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate,tridecyl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl(meth)acrylate, benzyl (meth)acrylate, glycidyl (meth)acrylate, allyl(meth)acrylate, phenoxydiethylene glycol (meth)acrylate,phenoxyhexaethylene glycol (meth)acrylate, dicyclopentenyl(meth)acrylate, isobornyl (meth)acrylate, phenyl (meth)acrylate,caprolactone-modified tetrahydrofurfuryl (meth)acrylate,caprolactone-modified dipentaerythritol (meth)acrylate,caprolactone-modified 2-hydroxyethyl (meth)acrylate, acrylamide andethylenediamine di(meth)acrylamide.

Examples of polymerizable compounds having at least one ethylenicallyunsaturated bond other than (meth)acrylic moieties are 1-alkenes, suchas 1-hexene, 1-heptene; branched alkenes, such as vinylcyclohexane,3,3-dimethyl-1-propene, 3-methyl-1-diisobutylene, 4-methyl-1-pentene;vinyl esters, such as vinyl acetate; styrene, substituted styreneshaving an alkyl substituent in the side chain, e.g. alpha-methylstyrene,substituted styrenes having an alkyl substituent on the ring, such asvinyltoluene and p-methylstyrene, halogenated styrenes, such asmonochlorostyrenes and dichlorostyrenes; or heterocyclic vinylcompounds, such as 2-vinylpyridine, 3-vinylpyridine,2-methyl-5-vinylpyridine, 3-ethyl-4-vinylpyridine,2,3-dimethyl-5-vinylpyridine, vinyl-pyrimidine, vinylpiperidine,9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole, 1-vinylimidazole,2-methyl-1-vinylimidazole, N-vinylpyrrolidone, 2-vinyl-pyrrolidone,N-vinylpyrrolidine, 3-vinylpyrrolidine, N-vinylcaprolactam,N-vinylbutyrolactam, vinyloxolane, vinylfuran; vinyl and isoprenylethers; maleic acid derivatives, such as maleic anhydride, methylmaleicanhydride, maleimide, methylmaleimide; and dienes, such asdivinylbenzene. The polymerizable compounds may be employed as amixture. They may be used to adjust the mechanical properties of thepolymerized light-cured dental material as the case requires.

It is possible to use a conventional photoinitiator such as camphorquinone in addition to the hexaaryl bisimidazole in the light-curabledental material of the invention. In this embodiment, the function ofthe hexaaryl bisimidazole may be an inhibitor, possibly in addition tothe function as a photoinitiator.

In one embodiment, the light-curable dental material is afiller-containing light-curable dental material, such as a dentalcomposite. In this embodiment, the light-curable dental material of theinvention comprises a solid particulate filler and a polymerizablematrix, wherein the polymerizable matrix comprises

(i) one or more polymerizable compounds each having at least oneethylenically unsaturated bond and(ii) a hexaaryl bisimidazole.

The light-curable dental composite of the invention (or otherfiller-containing light-curable dental material) contains a solidparticulate filler that provides strength to the polymerized dentalcomposite of the invention. The solid filler is a finely dividedparticulate material. The filler-containing light-curable dentalmaterial such as the dental composite contains at least 10% by weight,preferably at least 20% by weight, more preferably at least 50% byweight, and most preferably at least 70% by weight of said solid fillerbased on the total weight of said filler-containing light-curable dentalmaterial. The filler content may be expressed as volume-%, which makesthe numerical value of the filler content independent of the density ofsaid solid filler. Using this definition, the filler-containinglight-curable dental material such as the dental composite of theinvention contains generally at least 4% by volume, preferably at least8% by volume, more preferably at least 25% by volume, even morepreferred at least 35% by volume and most preferably at least 45% byvolume of said solid filler. Obviously, the exact amount of said solidfiller that can be incorporated into said polymerizable matrix dependson the size of the particles of said solid filler, i.e. on the surfacearea of said solid filler, and on the density of the filler.

Suitable fillers that may be used in the filler-containing light-curabledental material, notably the light-curable dental composite, includeorganic and inorganic solid fillers, whereby inorganic fillers arepreferred. Examples of inorganic fillers are glasses e.g. thosecontaining barium, strontium, boron, or zinc, aluminosilicate glass, andmetal oxides such as zinc oxide, zirconium oxide, aluminium oxide,silica, apatite, or a cured mixture of resin and filler ground orotherwise reduced in size to a powder. Other examples are fused silica,quartz, crystalline silica, amorphous silica, soda glass beads, glassrods, ceramic oxides, particulate silicate glass, radiopaque glasses(barium and strontium glasses), and synthetic minerals. It is alsopossible to employ finely divided materials and powderedhydroxyl-apatite, although materials that react with silane couplingagents are preferred. Also available as a filler are colloidal orsubmicron silicas coated with a polymer. As further examples of suitableinorganic fillers may be mentioned La₂O₃, ZrO₂, BiPO₄, CaWO₄, BaWO₄,SrF₂, Bi₂O₃. Suitable organic fillers include polymer granulates such aspolytetrafluoroethylene particles. Small amounts of pigments to allowmatching of the composition to various shades of teeth can be included.

The particles of said solid filler should have a mean size below 100 μm,preferably below 50 μm, more preferably below 20 μm. Two or more solidfillers may be mixed that differ in their mean particle size. Theparticle size distribution may be monomodal or may be polymodal.Preferably, the particle size distribution is bimodal, e.g. as describedin WO 2000/61073. The particles may be of any desired shape, forinstance spherical, irregular as is obtained by mechanical particle sizereduction, fibres, whiskers, platelets, dumbbell shaped, or cylindrical,and may be solid, hollow, or porous. Solid fillers that may be used inthe present invention are known in the art. Inorganic fillers arepreferably silanated before use in the present invention to render thesurface of the filler particles more hydrophobic. Silanating agents forthis purpose are well known in the art, e.g.3-methacryloxypropyltrimethoxysilane. Dental materials containing solidfillers of different particle sizes are for example described in WO00/61073.

The polymerizable matrix of the light-curable dental composite of theinvention may further comprise from 1.0 to 50, preferably from 1.0 to 15weight-% of at least one polymerizable monomer having a carboxylic acidgroup based on the total weight of said polymerizable matrix, saidpolymerizable monomer having a carboxylic acid group. Regarding saidpolymerizable monomer having a carboxylic acid group and the amounts ofit to be used in the polymerizable matrix of the light-curable dentalcomposite, reference is made to WO 2006/084769 that is included hereinby reference in its entirety.

The light-curable dental composite typically further contains a suitableco-initiator and an inhibitor as described above.

In another embodiment, the filler-containing light-curable dentalmaterial is a dental glass ionomer cement. Ionomer cements commonlycontain a polycarboxylic acid and an inorganic powder which reacts inthe presence of water by a curing reaction. Conventional ionomer cementsgenerally contain a powder component containing aluminosilicate and aliquid portion usually containing a polyacid such as polyacrylic acid,polymaleic acid, polyitaconic acid, or a copolymer of at least two ofthe acids, cf. “New Aspects of the Setting of Glass-Ionomer Cements,”Wasson et al., Journal of Dental Research; Vol. 72, No. 2, February,1993; pages 481-483. In glass ionomer cements, the primary reactionswhich cause the glass ionomer cement to harden is cross-linking ofpolycarboxylate chains by metal ions from the glass based on ionicforces. Moreover, during setting the acids of the glass ionomer cementdissolve the glass structure to release metal constituents of the glass.Ionic carboxylates of calcium, strontium and aluminum are mainly formedduring the setting process. In the present invention, the polymerizablematrix of the light-curable dental ionomer cement contains said one ormore polymerizable compounds each having at least one ethylenicallyunsaturated bond in addition to the polycarboxylic acid, and thehexaarylbisimidazole of the invention.

For dental ionomer cements, the fillers are particulate reactivefillers. A “particulate reactive filler” is a powdered metal oxide orhydroxide, mineral silicate, or ion leachable glass or ceramic, that iscapable of reacting with an ionomer in the presence of water to form ahydrogel. Examples of particulate reactive filler materials includematerials commonly known in the art of glass-ionomer cements such ascalcium or strontium-containing and aluminum-containing materials.Preferably, particulate reactive fillers contain leachable fluorideions. Specific examples of particulate reactive fillers are selectedfrom calcium alumino silicate glass, calcium alumino fluorosilicateglass, calcium aluminumfluoroborosilicate glass, strontiumaluminosilicate glass, strontium aluminofluorosilicate glass, strontiumaluminofluoroborosilicate glass. The glasses may also contain otherelements such as zinc, lanthanum, silver, copper, and iron. Suitableparticulate reactive fillers further include metal oxides such as zincoxide and magnesium oxide, and ion-leachable glasses, e.g., as describedin U.S. Pat. No. 3,655,605, U.S. Pat. No. 3,814,717, U.S. Pat. No.4,143,018, U.S. Pat. No. 4,209,434, U.S. Pat. No. 4,360,605, U.S. Pat.No. 4,376,835 and WO2005EP11584A.

The particulate reactive filler usually has an average particle size offrom 0.005 to 100 μm, preferably of from 0.01 to 40 μm as measuredusing, for example, by electron microscopy or by using a conventionallaser diffraction particle sizing method as embodied by a MALVERNMastersizer S or MALVERN Mastersizer 2000 apparatus. The particulatereactive filler may be a multimodal particulate reactive fillerrepresenting a mixture of two or more particulate fractions havingdifferent average particle sizes. The particulate reactive filler mayalso be a mixture of particles of different chemical composition. Inparticular, it is possible to use a mixture of a particulate reactivematerial and a particulate non-reactive material.

The light-curable dental ionomer cement typically further contains asuitable co-initiator and an inhibitor as described above. Regardingdental ionomer cements, the disclosure of WO 2006/013111 is incorporatedherein by reference in its entirety.

Experimental Part Measurement of Compressive and Yield Strengths

Metal forms with an internal diameter of 4 mm and a height of 6 mm asdescribed in ISO 9917 section 7.4 were used to prepare the specimens.The paste to be measured was filled into the forms, covered withpolyester foil, and pressed with metal plates to extrude excessmaterial. The material was then cured for 40 seconds from each end usinga dental curing lamp (Spectrum Lite, Dentsply) with an output between600 and 700 mW/cm². The forms complete with specimen were drawn acrosssilicon carbide paper (600 grit) until a smooth surface level with theend of the form was obtained, and then the cured specimens were removedfrom the form. The specimens were stored in water at 37° C. for 24 hoursbefore being tested in a universal testing machine (Zwick) with acrosshead speed of 1 mm/minute. Compressive strength was measured byloading the specimens to failure, and recording the maximum forcereached divided by the cross section area of the specimen. The stressstrain curve for each specimen was inspected and found to consistessentially of an initial straight portion followed by a curved portionleading to the final breaking point. The straight portion of the curvecorresponds to elastic behaviour of the material, whereas the curvedportion corresponds to plastic flow. The force at which the stressstrain curve first deviated from a straight line was taken as the yieldpoint. The yield point is expressed in MPa, and is calculated bydividing the yield force in Newtons by the cross-sectional area of thespecimen. The average value of at least five specimens for each materialwas calculated.

Measurement of Flexural Strength

Glass tubes with internal diameter 4 mm and length about 30 mm wherefilled with the composite material to a length of about 25 mm. The tubeswere placed in a LiCu Lite light oven (Dentsply) and hardened byexposure to the light for two minutes. After this the resulting hardenedcomposite cylinders were pushed from the tubes and stored in water at37° C. for 24 hours. The flexural strength was measured by testing thespecimens to failure in three-point bending mode using a Zwick universaltesting machine. The average value of at least five specimens for eachmaterial was calculated.

Measurement of Vickers Hardness (VH)

The composite material was filled into forms 8 mm diameter and 2 mmthick. The composite was hardened for 20 seconds with a dental curinglamp (Spectrum Lite, Dentsply) with an output between 600 and 700mW/cm². After this the specimens were removed from the forms and storedin water at 37° C. for 24 hours before being measured using a VickersHardness apparatus (Frank).

Lifetime and Depth of Cure

The lifetime (sensitivity to ambient light) of the materials wasmeasured at 10000 lux using the method given in ISO 4049:2000. Notehowever that ISO 4049:2000 specifies a lower light intensity of8000±1000 lux. The depth of cure of the materials was also measuredaccording to the method given in ISO 4049:2000.

-   1. Synthesis of bis 2-(2-chlorophenyl-4,5-diphenylimidazole),    (2Cl-HABI)-   1.1. The precursor 2-(2-chlorophenyl-4,5-diphenylimidazole) was    prepared according to a literature method (e.g. U.S. Pat. No.    3,784,557). Benzil (21 g) and ammonium acetate (60 g) were added to    glacial acetic acid (500 ml) and the mixture was stirred to dissolve    the benzil. 2-chlorobenzaldehyde (14 g) was then added and the    mixture was stirred and heated to 125° C. for 2 hours. The mixture    was allowed to cool and was then poured into water (1800 ml) with    stirring to give a pale yellow viscous mass. This gradually    solidified to give a pale yellow solid which was filtered and dried    (39 g). The solid was dissolved in boiling ethanol (400 ml) and    after cooling, filtering, and drying gave an off-white slightly    yellow fluorescing powder (18.3 g).-   1.2. Bis 2-(2-chlorophenyl-4,5-diphenylimidazole) was prepared from    the product of 1.1 by oxidation with aqueous potassium ferricyanide,    as described in J. Org. Chem 36, 16, 1971, 2262-2266. The product    from 1.1 (4.95 g) was slurried with toluene (150 ml). A solution of    potassium ferricyanide (9.9 g), and sodium hydroxide (6.0 g) in    water (100 ml) was added to the toluene slurry, and the two phases    stirred vigorously together. The solid dissolved over about one hour    after which a mauve organic phase had been formed. The mixture was    stirred together for about 18 hours after which the organic layer    was separated and dried over anhydrous MgSO₄. The organic layer was    a yellow colour when stored in the dark, but turned mauve after a    short exposure to light. The toluene was removed under vacuum to    give a viscous yellow liquid. Diethyl ether (50 ml) was added and    precipitation initiated by scratching inside the flask with a glass    rod. After 2 hours the ether was decanted to leave yellow crystals    (4.6 g). H¹NMR analysis showed the crystals to be an approximately    1:1 complex between ether and bis    2-(2-chlorophenyl-4,5-diphenylimidazole). The crystals were stored    at 40° C. under vacuum after which pure bis    2-(2-chlorophenyl-4,5-diphenylimidazole) was obtained (4.44 g).-   2. Synthesis of Bis 2-(2,4-dichlorophenyl-4,5-diphenylimidazole),    (2,4-Cl HABI) 2-(2,4-dichlorophenyl-4,5-diphenylimidazole) and bis    2-(2,4-dichlorophenyl-4,5-diphenylimidazole) were synthesised    analogously to the methods in 1 above by substituting the    2-chlorobenzaldehyde by a molar equivalent of    2,4-dichlorobenzaldehyde. Benzil (21 g) and ammonium acetate (60 g)    were dissolved in glacial acetic acid (400 ml).    2,4-dichlorobenzaldehyde was then added and the mixture stirred and    heated to reflux for 2 hours. After cooling, the mixture was poured    into cold water (1500 ml) with vigorous stirring. The white    precipitate was filtered off and dried in vacuum to give 97 g. This    was refluxed with a mixture of ethanol (350 ml) and acetic acid (150    ml), filtered hot, and then allowed to cool. The white crystals were    filtered off and dried to give (21.1 g) of    2-(2,4-dichlorophenyl-4,5-diphenylimidazole). Water was slowly added    to the filtrate until this just turned cloudy. After cooling in a    refrigerator, a further 6.6 g of crystals were obtained. The    2-(2,4-dichlorophenyl-4,5-diphenylimidazole) was oxidised to the    dimer bis 2-(2,4-dichlorophenyl-4,5-diphenylimidazole) following the    procedure given in method 1.2 for bis    2-(2-chlorophenyl-4,5-diphenylimidazole), except that the toluene    was replaced by dichloromethane as solvent.-   3. Synthesis of Bis(2,4,5-triphenylimidazole), also known as Lophine    dimer, or HABI 2,4,5-triphenylimidazole and    bis(2,4,5-triphenylimidazole) were prepared analogously to the    method in 2 above, by substituting the 2,4-dichlorobenzaldehyde by a    molar equivalent of benzaldehyde.-   4. 2-(2-methoxyphenyl-4,5-diphenylimidazole) and bis Bis    2-(2-methoxyphenyl-4,5-diphenylimidazole) were synthesised    analogously to the method in 2 by substituting the    2,4-dichlorobenzaldehyde for a molar equivalent of    2-methoxybenzaldehyde.-   5. Further, differently substituted hexaarylbisimidazoles were    synthesised analogously to the method in 1 by exchanging the    2-chlorobenzaldehyde by a molar equivalent of the appropriately    substituted aldehyde, for example 4-methoxybenzaldehyde,    2,4-dimethoxybenzaldehyde, 4-cyanobenzaldehyde, naphthaldehyde,    2-trifluoromethyl benzaldehyde, and the like.-   6. Yet further substituted hexaarylbisimidazoles were synthesised    analogously to the method in 1 by exchanging benzil by a molar    equivalent of the appropriately substituted benzil, for example    4,4′-dimethoxybenzil, 4,4′-dimethylbenzil,    2-chloro-3′,4′-dimethoxybenzil, and the like. The same approach may    be taken for preparing hexaarylbisimidazoles having one or more    heteroaromatic aryl groups.

EXAMPLES 1 TO 23

Preparation of experimental resin mixtures: Resin mixtures were preparedwith initiator and co-initiator concentrations as given in Table 2 byfirst mixing together UDMA (40 parts), HPGM (7 parts), and EBA (53parts) at 40° C. for Resin A, or UDMA (5 parts), HPGM (5 parts), TMPTMA(5 parts), and EBA (85 parts) for Resin B. The initiators and inhibitorswere then added as required in amounts shown in the table, and dissolvedby stirring at 40-50° C. until no solid particles remained.

Preparation of dental composites: The chosen resin mixture (33.0 g),Aerosil R972 (0.75 g), and silanated glass with a mean particle size ofabout 0.8 μm (116.25 g) were kneaded together at 40° C. in a verticalkneader for 160 minutes, and the resulting paste was then degassed bystirring for ten minutes at a pressure of 210±10 mbar. Physicalproperties of the pastes were measured as described below, and are shownin Table 2.

TABLE 2 Formulations comprising hexaarylbisimidazoles, with measuredphysical properties co- % co- comp. yield flex. life- Ex. initiatorinitiator initiator initiator BHT str. str. str. time Doc No. resin — %— % % MPa MPa MPa VH5 sec mm 1 A 2-Cl HABI 0.5 MMT 0.5 0.0 327.5 152 13967 300 2.0 2 A 2-Cl HABI 0.5 MMT 0.25 0.0 325.6 150 138.5 60 480 1.8 3 A2-Cl HABI 0.25 MMT 0.5 0.0 302.9 136 131.5 54 540 1.8 4 A 2-Cl HABI 0.25MMT 0.25 0.0 284.6 124 141.4 55 600 1.6 5 A 2-Cl HABI 0.75 MMT 0.25 0.0348.5 163 138.7 63 360 1.9 6 A 2,4-Cl HABI 0.5 MMT 0.5 0.0 322.0 175142.5 59 180 2.0 7 A 2,4-Cl HABI 0.5 MMT 0.25 0.0 283.5 152 141.1 57 4203.0 8 A 2,4-Cl HABI 0.25 MMT 0.5 0.0 260.6 131 122.0 49 480 2.0 9 B 2-ClHABI 0.5 MMT 0.5 0.05 253.2 110 93.0 23 >300 1.6 10 B TCDM-HABI 0.5 MMT0.5 0.05 280.9 — 89.1 — >300 1.8 11 B 2,4-Cl HABI 0.5 MMT 0.5 0.05 267.2127 124.6 — >300 2.0 12 B 2,4-Cl HABI 0.5 MMT 0.75 0.05 317.5 156 150.257 >300 2.3 13 B 2-Cl HABI 0.66 MMT 0.35 0.05 272.4 131 132.6 41 >3002.0 14 B TCDM-HABI 0.75 MMT 0.35 0.05 271.5 120 122.4 28 >300 1.9 15 B2,4-Cl HABI 0.73 MMT 0.35 0.05 277.0 138 140.0 47 >300 2.24 16 B 2-TFMHABI 0.73 MMT 0.35 0.05 281.5 135 137.9 33 >300 2.06 17 B 2-Cl HABI 0.66MBT 0.51 0.05 299.4 152 123.2 55 >300 2.3 18 B TCDM-HABI 0.75 MBT 0.510.05 208.5 — — 54 >300 1.6 19 B 2,4-Cl HABI 0.73 MBT 0.51 0.05 297.3 149122.1 59 >300 2.3 20 B 2-TFM HABI 0.73 MBT 0.51 0.05 293.9 144 144.954 >300 2.1 21 B 2,4-Cl, HABI 0.73 MBT 0.76 0.05 319.6 160 150.7 58 2803.4 22 B 2-TFM HABI 0.73 MBT 0.76 0.05 318.2 156 161.9 59 210 2.5 23 B2,4Cl HABI/ 0.73/ DMABE 0.6 0.3 284.4 136 127 49 >300 — CQ 0.31

Comparative Examples

co- % co- comp. yield flex. life- Initiator initiator initiatorinitiator BHT str. str. str. time Doc No. Resin — % — % % MPa MPa MPaVH5 sec mm 1 A CQ 0.31 DMABE 0.6 0.6 293.5 133 119.7 58 135 3.0 2 B CQ0.31 DMABE 0.6 0.6 264.8 145 100.7 58 70 3.6

KEY TO ABBREVIATIONS

-   UDMA Urethane dimethacrylate-   HPGM the reaction product of Hydroxypropyl methacrylate and glutaric    anhydride-   EBA Ethoxylated bisphenol A dimethacrylate-   CQ Camphorquinone-   DMABE Dimethylamino benzoic acid, ethyl ester-   BHT butylated hydroxytoluene-   MMT 3-Mercapto-4-methyl-4H-1,2,4-triazole-   2-Cl HABI    2,2′-Bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,1′-biimidazole-   2,4-Cl HABI    2,2′-Bis(2,4-dichlorophenyl)-4,4′,5,5′-tetraphenyl-1,1′-biimidazole-   TCDM    2,2,′4-Tris(2-chlorophenyl)-5-(3,4-dimethoxyphenyl)-4′,5′-diphenyl-1,1′-biimidazole-   2-TFM HABI    2,2′-Bis(2-trifluoromethyl)-4,4′,5,5′-tetraphenyl-1,1′-biimidazole-   CDM 2-(o-Chloro)-4,5-bis(m-methoxyphenyl)-1,1′-biimidazole-   TCTM 2,5-Bis(o-chlorophenyl)-4-[3,4-dimethoxyphenyl]-1H-imidazole    dimer-   2-MBO 2-Mercaptobenzoxazole-   2-MBT 2-Mercaptobenzothiazole-   Doc depth of cure-   VH5 Vickers hardness measured with a load of 5 kg (49.03 N)

It is seen from the examples in Table 2 that the use of a bisimidazoleinitiator according to the present invention leads to lengthenedlifetimes of the compositions under the influence of ambient light at10000 lux. Further, composite material with improved compressive, yield,and flexural strengths compared to the comparative examples can beobtained.

The content (including description and claims) of European patentapplication 08008693.7 filed on May 8, 2008, the priority of which isclaimed, is included herein by reference in its entirety.

1. A light-curable dental material comprising (i) a polymerizablecompound having at least one ethylenically unsaturated bond and (ii) ahexaaryl bisimidazole.
 2. The light-curable dental material according toclaim 1, further comprising an α,β-diketone as a polymerizationinitiator and optionally a polymerisation inhibitor.
 3. Thelight-curable dental material according to any one of claims 1 or 2,further comprising a solid particulate filler.
 4. The light-curabledental material according to any of claims 1 to 3, wherein said hexaarylbisimidazole has the following structure (I):Ar₃T-TAr₃  (I) wherein each T is an imidazole moiety and each Ar standsfor an optionally substituted aryl group that may be the same ordifferent and wherein the substitution on different groups Ar may be thesame or different.
 5. The light-curable dental material according toclaim 1 or 4, wherein the hexaaryl bisimidazole is a homodimer or aheterodimer obtainable by oxidation of any of the triarylimidazolesrepresented by the following formula:

wherein Ar¹, Ar², and Ar³ may be the same or different and eachrepresents an optionally substituted aryl group and multiple Ar¹, Ar²,or Ar³ on the same hexaarylbisimidazole may be the same or different. 6.The light-curable dental material according to any one of claims 1 to 5,wherein the hexaaryl bisimidazole has the structure of the followingformula:

wherein Ar¹, Ar² and Ar³ may be the same or different and eachrepresents an optionally substituted aryl group and multiple Ar¹, Ar² orAr³ on the same hexaarylbisimidazole compound may be the same ordifferent.
 7. The light-curable dental material according to any one ofclaims 4 to 6, wherein the aryl group is selected from phenyl andnaphthyl.
 8. The light-curable dental material according to any one ofclaim 4 to 7, wherein each aryl group may have 1 to 5 substituentsselected from the group consisting of C₁₋₆-alkyl, C₁₋₆-alkoxy, hydroxy,halo, cyano, nitro, nitroso, mercapto, carboxyl, sulfonate, thiol,amino, phenyl, pyridyl, and trifluoromethyl.
 9. The light-curable dentalmaterial according to any one of claim 1 to 8, wherein the concentrationof said hexaaryl bisimidazole based on the total weight of allpolymerizable compounds having at least one ethylenically unsaturatedbond is from 0.01 to 2.0, preferably from 0.05 to 1.5, and mostpreferably from 0.1 to 1.0 weight-%.
 10. The light-curable dentalmaterial according to any one of claim 1 to 9, further comprising aco-initiator selected from the following compound classes: thiols,heteroaromatic thiols, benzothiazoles, benzooxazoles, tertiary amines,alcohols, thiocarboxylic acids.
 11. The light-curable dental materialaccording to any one of claim 1 to 10, further comprising a sensitizerfor extending the wavelength range usable for activating the hexaarylbisimidazole.
 12. The light-curable dental material according to any oneof claims 1 to 11, wherein the light-curable dental material is selectedfrom the group consisting of a dental composite, dental ionomer cement,dental sealant, dental adhesive, dental adhesion promoter, dentaladhesion preventer, dental cement, dental crown-forming material, anddental impression material.
 13. Light-cured dental material obtained orobtainable by light curing the light-curable dental material of any oneof claims 1 to
 12. 14. Use of a hexaaryl bisimidazole in a light-curabledental material as a photoinitiator.
 15. Light-curable dental materialsuch as dental composite comprising a solid particulate filler and apolymerizable matrix, wherein the polymerizable matrix comprises: (i)one or more polymerizable compounds each having at least oneethylenically unsaturated bond and (ii) a hexaaryl bisimidazole.